Referring to FIG. 1, an existing large-diameter underwater pipe for used in pumping seawater is made into a plurality of pipes 1 using a PE material. Two adjacent pipes 1 are welded at their corresponding ends using a welding equipment (not shown) to obtain the desired length. The pipes 1 are pressed toward each other along an axial direction so as to be welded.
The structure of an existing welding equipment is rather complicated, and a welded junction of the two pipes 1 has an uneven shape, so that turbulence and resistance are prone to occur during pumping of the seawater. Furthermore, the structural strength of the welded junction of the two adjacent pipes 1 is insufficient, so that it is susceptible to torque and pressure, thereby generating breakage.